1,883 research outputs found

### alpha-cluster states in intermediate mass nuclei

Properties of intermediate mass nuclei have been investigated within the
framework of the alpha-cluster model in combination with systematic
double-folding potentials. Previously, this alpha-cluster model has been widely
applied to light nuclei, in particular to 8Be = alpha \otimes alpha, 20Ne = 16O
\otimes alpha, and 44Ti = 40Ca \otimes alpha, and to heavy nuclei, in
particular to 212Po = 208Pb \otimes alpha. In the present work a wide range of
nuclei is investigated with the magic neutron number N = 50 in the mass range
around A \approx 80 - 100: (A+4,N=52) = (A,N=50) \otimes alpha. It is found
that excitation energies, decay properties, and transition strengths can be
described successfully within this model. The smooth and small variation of the
underlying parameters of the alpha-nucleus potential may be used for
extrapolations to predict experimentally unknown properties in the nuclei under
study.Comment: 9 pages, 7 figures, TONPPJ, accepte

### Nucleosynthesis of $^{92}$Nb and the relevance of the low-lying isomer at 135.5 keV

Background: Because of its half-life of about 35 million years, 92Nb is
considered as a chronometer for nucleosynthesis events prior to the birth of
our sun. The abundance of 92Nb in the early solar system can be derived from
meteoritic data. It has to be compared to theoretical estimates for the
production of 92Nb to determine the time between the last nucleosynthesis event
before the formation of the early solar system.
Purpose: The influence of a low-lying short-lived isomer on the
nucleosynthesis of 92Nb is analyzed. The thermal coupling between the ground
state and the isomer via so-called intermediate states affects the production
and survival of 92Nb.
Method: The properties of the lowest intermediate state in 92Nb are known
from experiment. From the lifetime of the intermediate state and from its decay
branchings, the transition rate from the ground state to the isomer and the
effective half-life of 92Nb are calculated as a function of the temperature.
Results: The coupling between the ground state and the isomer is strong. This
leads to thermalization of ground state and isomer in the nucleosynthesis of
92Nb in any explosive production scenario and almost 100% survival of 92Nb in
its ground state. However, the strong coupling leads to a temperature-dependent
effective half-life of 92Nb which makes the 92Nb survival very sensitive to
temperatures as low as about 8 keV, thus turning 92Nb at least partly into a
thermometer.
Conclusions: The low-lying isomer in 92Nb does not affect the production of
92Nb in explosive scenarios. In retrospect this validates all previous studies
where the isomer was not taken into account. However, the dramatic reduction of
the effective half-life at temperatures below 10 keV may affect the survival of
92Nb after its synthesis in supernovae which are the most likely astrophysical
site for the nucleosynthesis of 92Nb.Comment: 5 pages, 3 figures; Phys. Rev. C, accepted for publicatio

### $\alpha$-cluster states in $^{46,54}$Cr from double-folding potentials

$\alpha$--cluster states in $^{46}$Cr and $^{54}$Cr are investigated in the
double-folding model. This study complements a recent similar work of Souza and
Miyake \cite{Sou17} which was based on a specially shaped potential. Excitation
energies, reduced widths, intercluster separations, and intra-band transition
strengths are calculated and compared to experimental values for the ground
state bands in $^{46}$Cr and $^{54}$Cr. The $\alpha$-cluster potential is also
applied to elastic scattering at low and intermediate energies. Here, as a
byproduct, a larger radial extent of the neutron density in $^{50}$Ti is found.Comment: 9 pages, 7 figures, Europ. Phys. J. A, accepted for publicatio

### Photon-induced Reactions in Stars and in the Laboratory: A Critical Comparison

Photon-induced reactions during the astrophysical p- (or gamma-) process
occur at typical temperatures of 1.8 < T9 < 3.3. Experimental data of
(gamma,n), (gamma,p), or (gamma,alpha) reactions - if available in the relevant
energy region - cannot be used directly to measure astrophysical (gamma,n),
(gamma,p), or (gamma,alpha) reaction rates because of the thermal excitation of
target nuclei at these high temperatures. Usually, statistical model
calculations are used to predict photon-induced reaction rates. The relations
between experimental reaction cross sections, theoretical predictions, and
astrophysical reaction rates will be critically discussed.Comment: 8 pages, 2 figures, Proc. Tours Symposium Nuclear Physics V 2003,
p.53

### Uncertainty of the astrophysical $^{17,18}$O($\alpha$,n)$^{20,21}$Ne reaction rates and the applicability of the statistical model for nuclei with $A \lesssim 20$

Background: The ($\alpha$,n) and ($\alpha$,$\gamma$) reactions on $^{17,18}$O
have significant impact on the neutron balance in the astrophysical
$s$-process. In this scenario stellar reaction rates are required for
relatively low temperatures below $T_9 \lesssim 1$.
Purpose: The uncertainties of the $^{17,18}$O($\alpha$,n)$^{20,21}$Ne
reactions are investigated. Statistical model calculations are performed to
study the applicability of this model for relatively light nuclei in extension
to a recent review for the $20 \le A \le 50$ mass range.
Method: The available experimental data for the
$^{17,18}$O($\alpha$,n)$^{20,21}$Ne reactions are compared to statistical model
calculations. Additionally, the reverse $^{20}$Ne(n,$\alpha$)$^{17}$O reaction
is investigated, and similar studies for the $^{17}$F mirror nucleus are
provided.
Results: It is found that on average the available experimental data for
$^{17}$O and $^{18}$O are well described within the statistical model,
resulting in reliable reaction rates above $T_9 \gtrsim 1.5$ from these
calculations. However, significant experimental uncertainties are identified
for the $^{17}$O($\alpha$,n$_0$)$^{20}$Ne(g.s.) channel.
Conclusions: The statistical model is able to predict astrophysical reaction
rates for temperatures above 1 GK with uncertainties of less than a factor of
two for the nuclei under study. An experimental discrepancy for the
$^{17}$O($\alpha$,n)$^{20}$Ne reaction needs to be resolved.Comment: 10 pages, 9 figures, Phys. Rev. C, accepted for publicatio

### Calculation of Hydrogenic Bethe Logarithms for Rydberg States

We describe the calculation of hydrogenic (one-loop) Bethe logarithms for all
states with principal quantum numbers n <= 200. While, in principle, the
calculation of the Bethe logarithm is a rather easy computational problem
involving only the nonrelativistic (Schroedinger) theory of the hydrogen atom,
certain calculational difficulties affect highly excited states, and in
particular states for which the principal quantum number is much larger than
the orbital angular momentum quantum number. Two evaluation methods are
contrasted. One of these is based on the calculation of the principal value of
a specific integral over a virtual photon energy. The other method relies
directly on the spectral representation of the Schroedinger-Coulomb propagator.
Selected numerical results are presented. The full set of values is available
at quant-ph/0504002.Comment: 10 pages, RevTe

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